Spindle pole bodies (SPB) from S. cerevisiae are the microtubule organizing centers of yeast. This is a much simpler system than the mammalian centrosomes to study the central question of how cells utilize microtubule organizi~ng centers for all aspects of cell division. The STEM is being used to determine the mass of the yeast SPB. It is known to be a large cylinder spanning the nuclear envelope. The diameter can range from 100 nm to over 200 nm, with an invariant height of about 100 nm. There are three major size classes. An important question is whether the size classes correspond to distinct masses, which would indicate a quantized addition of proteins. The SPB core is being used for 3-') electron tomography. Heparin treatment of the SPB disassembles the microtubule and removes the outer and inner plaques. STEM data from a time course of heparin treatment confirmed a metastable state for the core SPB of approximately 0.5 gigadaltons (g')). Masses of the whole SPB range from 0.6 to several g'), extending the range of STEM mass measurements to a higher range. The masses are clearly falling into size classes, and flirther data and analysis is underway to confirm this. Fixation of whole SPBs is giving much better preserved specimens.